Transparent ultraviolet photodetector

ABSTRACT

A method of fabricating a visibly transparent, ultraviolet (UV) photodetector is provided. The method includes laying a first electrode onto a substrate surface, the first electrode being formed of a carbon-based, single-layer material. A block is patterned over an end of the first electrode and portions of the substrate surface. The block is formed of a visibly transparent material that is able to be deposited into the block at 75° C.-125° C. In addition, the method includes masking a section of the block and exposed sections of the first electrode. A second electrode is laid onto an unmasked section of the block with an end of the second electrode laid onto the substrate surface. The second electrode is formed of the carbon-based, single-layer material.

DOMESTIC PRIORITY

This application is a continuation application of U.S. application Ser.No. 15/994,510, titled “TRANSPARENT ULTRAVIOLET PHOTODETECTOR”, whichwas filed on May 31, 2018. The entire disclosures of U.S. applicationSer. No. 15/994,510 are incorporated herein by reference.

BACKGROUND

The present invention generally relates to transparent ultravioletphotodetectors. More specifically, the present invention relates to atransparent ultraviolet photodetector configured to achieve acombination of low temperature processing and visible lighttransparency.

Photo-sensors or photodetectors are sensors that detect or sense lightand/or other electromagnetic energy. A photodetector typically operatesby converting incident light (i.e., photons) into charged carriers.These charged carriers are then directed into electrodes as current thatis used to identify when photo detection has occurred. Photodetectorsare used in a variety of applications, including, for example,applications involving plastic and flexible substrate platforms.

SUMMARY

Embodiments of the present invention are directed to a method offabricating a visibly transparent, ultraviolet (UV) photodetector. Anon-limiting example of the method includes laying a first electrodeonto a substrate surface, the first electrode being formed of acarbon-based, single-layer material. A block is patterned over an end ofthe first electrode and portions of the substrate surface. The block isformed of a visibly transparent material that is able to be depositedinto the block at 75° C.-125° C. In addition, the method includesmasking a section of the block and exposed sections of the firstelectrode. A second electrode is laid onto an unmasked section of theblock with an end of the second electrode laid onto the substratesurface. The second electrode is formed of the carbon-based,single-layer material.

Embodiments of the present invention are directed to method offabricating a visibly transparent, ultraviolet (UV) photodetector. Anon-limiting example of the method includes laying a first grapheneelectrode between a substrate surface and a first conductive contact. Azinc oxide (ZnO) block is patterned over an end of the first grapheneelectrode and proximal portions of the substrate surface at a distancefrom the first conductive contact. The non-limiting example of themethod further includes masking complementary sections of the firstconductive contact and the ZnO block and masking a region definedbetween the complementary sections of the first conductive contact andthe ZnO block. A second graphene electrode is laid onto an unmaskedsection of the ZnO block with an end of the second graphene electrodebeing laid onto the substrate surface. A second conductive contact isassembled onto the end of the second graphene electrode.

Embodiments of the invention are directed to a visibly transparent,ultraviolet (UV) photodetector. A non-limiting example of the visiblytransparent, UV photodetector includes a visibly transparent stackassembled on a substrate surface. In the non-limiting example of thevisibly transparent, UV photodetector, the visibly transparent stackincludes a zinc oxide (ZnO) block interleaved between first and secondgraphene electrodes.

Additional technical features and benefits are realized through thetechniques of the present invention. Embodiments and aspects of theinvention are described in detail herein and are considered a part ofthe claimed subject matter. For a better understanding, refer to thedetailed description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The specifics of the exclusive rights described herein are particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe embodiments of the invention are apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 is a side view of a first graphene electrode assembled between aconductive contact and a substrate surface in accordance withembodiments of the invention;

FIG. 2 is a top-down view of the first graphene electrode, theconductive contact and the substrate surface of FIG. 1;

FIG. 3 is a side view of a zinc oxide (ZnO) block formed on the firstgraphene electrode of FIGS. 1 and 2 in accordance with embodiments ofthe invention;

FIG. 4 is a top-down view of the ZnO block formed on the first grapheneelectrode of FIG. 3;

FIG. 5 is a side view of a mask formed between the conductive contactand the ZnO block of FIGS. 3 and 4 in accordance with embodiments of theinvention;

FIG. 6 is a top-down view of the mask of FIG. 5;

FIG. 7 is a side view of a second graphene electrode formed on the ZnOblock and the substrate surface of FIGS. 5 and 6 in accordance withembodiments of the invention;

FIG. 8 is a top-down view of the second graphene electrode of FIG. 7;

FIG. 9 is a side view of an operation of a photodetector in accordancewith embodiments of the invention; and

FIG. 10 is a flow diagram illustrating a method of fabricating a visiblytransparent, ultraviolet (UV) photodetector in accordance withembodiments.

The diagrams depicted herein are illustrative. There can be manyvariations to the diagram or the operations described therein withoutdeparting from the spirit of the invention. For instance, the actionscan be performed in a differing order or actions can be added, deletedor modified. Also, the term “coupled” and variations thereof describeshaving a communications path between two elements and does not imply adirect connection between the elements with no interveningelements/connections between them. All of these variations areconsidered a part of the specification.

In the accompanying figures and following detailed description of thedescribed embodiments, the various elements illustrated in the figuresare provided with two or three digit reference numbers. With minorexceptions, the leftmost digit(s) of each reference number correspond tothe figure in which its element is first illustrated.

DETAILED DESCRIPTION

For the sake of brevity, conventional techniques related tosemiconductor device, integrated circuit (IC) and photodetectorfabrication may or may not be described in detail herein. Moreover, thevarious tasks and process steps described herein can be incorporatedinto a more comprehensive procedure or process having additional stepsor functionality not described in detail herein. In particular, varioussteps in the manufacture of semiconductor devices, semiconductor-basedICs and photodetectors are well known and so, in the interest ofbrevity, many conventional steps will only be mentioned briefly hereinor will be omitted entirely without providing the well-known processdetails.

Turning now to an overview of technologies that are more specificallyrelevant to aspects of the invention, optically transparent ultraviolet(UV) photodetectors are conventionally made of wide band gap materials.These include, but are not limited to, silicon carbide (SiC), galliumphosphide (GaP) and gallium nitride (GaN). Relative to ZnO deposition,which can be accomplished at temperatures as low as 75° C.-125° C.,deposition and/or growth of SiC, GaP and GaN are high temperatureprocesses. Furthermore, GaP and GaN are less visibly transparent thanZnO within the visible light range (390 nm-700 nm). For comparison, at400 nm, GaP loses all of its transparency while GaN loses approximately75% of its transparency.

Turning now to an overview of the aspects of the invention, one or moreembodiments of the invention address the above-described shortcomings ofthe prior art by presenting a pathway for fabricating visiblytransparent, ultraviolet (UV) photodetectors with low temperatureprocesses. The method utilizes a first material, such as graphene oranother suitable material, as an electrode material and a secondmaterial, such as zinc oxide (ZnO) or another suitable material, as amain UV absorbing material to realize the photodetecting device. Thefirst material (i.e., the graphene) is disposed in layers and providesfor light transmission at 400 nm beyond what would be possible withconventional photodetectors. The first material (i.e., the graphene) isalso transferred, instead of grown or deposited, onto the substrate ofinterest in an inherently low temperature process.

The photodetector having been made of the first and second materials(i.e., the graphene and the ZnO) will allow for a combination of lowtemperature processing and visible light transparency that is notachievable with other commonly-used materials. These characteristicsmake the proposed photodetector a candidate for applications involvingplastic and flexible substrate platforms.

The above-described aspects of the invention address the shortcomings ofthe prior art in that they present a practical scheme to fabricatevisibly transparent UV photodetectors based on graphene and ZnO. Theresulting visibly transparent UV photodetectors have a structure inwhich incident UV radiation is absorbed by the second material (i.e.,the ZnO) and an excited charge is transferred to electrodes of the firstmaterial (i.e., the graphene). This leads to a detectable increase incurrent through the device. The fabrication scheme includes a depositionof the second material onto the first material (i.e., a deposition ofthe ZnO onto the graphene) by atomic layer deposition with another layerof the first material placed on top of the second material (i.e., thegraphene placed on top of the ZnO). This forms a visibly transparentfirst material/second material/first material (i.e.,graphene/ZnO/graphene) stack.

For purposes of clarity and brevity, the following description willgenerally refer to the first material as being graphene and to thesecond material as being ZnO. This is done for clarity and brevity butit is to be understood that the first and second materials need not belimited to graphene and ZnO and that the disclosure as a whole is notlimited to the graphene and ZnO embodiments.

Turning now to a more detailed description of aspects of the presentinvention, FIG. 1 is a side view of a substrate 10 having an uppersurface 11, a first conductive contact 12 and a first graphene electrode13 laid between the first conductive contact 12 and the upper surface 11of the substrate 10. The substrate 10 and the upper surface 11 can beformed of visibly transparent materials including, but not limited to,visibly transparent glass and visibly transparent plastic. As usedherein, visible transparency can refer, for example, to a property of amaterial whereby the material transmits light without appreciablescattering so that bodies lying beyond are visible to the human eye. Thefirst graphene electrode 13 is positioned or laid on the upper surface11 in an inherently low-temperature process. This inherentlylow-temperature process can be executed with respect to an entirety ofthe first graphene electrode 13 as a process that is selected from thegroup of processes consisting of mechanical exfoliation and a transferprocess.

The first graphene electrode 13 can be initially grown on a copper foil.Once the first graphene electrode 13 is completely grown, the graphenesurface can be coated with a layer of polymer resist, and the copperfoil can be etched with the polymer resist providing structural support.The first graphene electrode 13 and the polymer resist are thentransferred to a water-based solution. The first graphene electrode 13and the polymer resist are then removed from the water-based solutionand transferred to the substrate 10. The first graphene electrode 13 canbe doped with either n-type or p-type dopants.

With reference to FIG. 2, once the first graphene electrode 13 ispositioned or laid onto the upper surface 11, the first grapheneelectrode 13 can be formed into a first graphene electrode strip 130that extends from an edge 110 of the upper surface 11 along asignificant length of the upper surface 11. Formation of the firstgraphene electrode 13 into the first graphene electrode strip 130 can beexecuted by the formation of a mask, which can include polymethylmethacrylate (PMMA) resist material that is patterned by electron-beamlithography techniques, and a subsequent etch of sections of the firstgraphene electrode 13 that are exposed by the mask. The subsequent etchcan be a reactive ion etch (RIE). Following completion of the etchprocess, the mask can be removed by, for example dissolution in acetonefor PMMA resist material.

The first conductive contact 12 can include a material selected from thegroup consisting of titanium (Ti), palladium (Pd) and gold (Au). Thefirst conductive contact 12 can be patterned and deposited on an end ofthe first graphene electrode strip 130 proximate to or coplanar with theedge 111 of the upper surface 11 by, for example, PMMA-based lift-offlithography. As shown in FIG. 2, a width of the first graphene electrodestrip 130 is less than a corresponding width of the first conductivecontact 12.

With reference to FIGS. 3 and 4, a ZnO block 14 is formed on an uppersurface 131 of the first graphene electrode strip 130 at a distance fromthe first conductive contact 12. With the ZnO block 14 formed at thedistance from the first conductive contact 12, respective complementarysections 120 and 140 of the first conductive contact 12 and the ZnOblock 14 are separated by a distance D. The distance D defines a region1214 between the first conductive contact 12 and the ZnO block 14through which a corresponding section of the first graphene electrodestrip 130 is exposed. The ZnO block 14 can be patterned and depositedby, for example, atomic layer deposition in a relatively low-temperatureenvironment of between 75° C.-125° C. and PMMA-based lift-offlithography.

In some cases, atomic layer deposition (ALD) can be used to form the ZnOblock 14. In such cases, the inert upper surface 131 of the firstgraphene electrode strip 13 could be required to be functionalized witha seed molecule layer to promote nucleation (see block 1003 of FIG. 10).The seed molecule could be nitrogen dioxide or another suitablemolecule.

In accordance with alternative embodiments of the invention, it is to beunderstood that blocks of other materials could be used in addition toor instead of the ZnO block 14. For example, an indium tin oxide (ITO)block could be used.

As shown in FIG. 4, a width of the first graphene electrode strip 130 isless than a corresponding width of the ZnO block 14. In some cases, therespective widths of the first conductive contact 12 and the ZnO block14 can be substantially similar.

With reference to FIGS. 5 and 6, a mask 15 is formed between therespective complementary sections 120 and 140 of the first conductivecontact 12 and in the region 1214 such that the corresponding section ofthe first graphene electrode strip 130 is masked. The mask 15 can be aninsulating hard mask and can be deposited by, for example, PMMA-basedlift-off lithography. As shown in FIGS. 5 and 6, a width of the mask 15is greater than a corresponding width of the first graphene electrodestrip 130 and less than the respective corresponding widths of the firstconductive contact 12 and the ZnO block 14. As such, in the region 1214,the upper surface 131 and the sidewalls of the first graphene electrodestrip 130 at the corresponding section of the first graphene electrodestrip 130 are masked by the mask 15.

With reference to FIG. 7, a second graphene electrode 16 is positionedor laid on an upper surface 141 of the ZnO block 14 and on the substratesurface 11 in an inherently low-temperature process. This inherentlylow-temperature process can be executed with respect to an entirety ofthe second graphene electrode 16 as a process that is selected from thegroup of processes consisting of a mechanical exfoliation process and atransfer process.

The second graphene electrode 16 can be initially grown on a copperfoil. Once the second graphene electrode 16 is completely grown, thegraphene surface can be coated with a layer of polymer resist, and thecopper foil can be etched with the polymer resist providing structuralsupport. The second graphene electrode 16 and the polymer resist arethen transferred to a water-based solution. The second grapheneelectrode 16 and the polymer resist are then removed from thewater-based solution and transferred to the ZnO block 14. The secondgraphene electrode 16 can be doped with either n-type or p-type dopants.

With reference to FIG. 8, once the second graphene electrode 16 ispositioned or laid onto the upper surface 141 of the ZnO block 14 andthe upper surface 11, the second graphene electrode 16 can be formedinto a second graphene electrode strip 160 that extends from the uppersurface 141, along a connective section 161 thereof, which extends at anangle from the upper surface 141 to the upper surface 11, and along anend section 162 thereof, which extends along the upper surface 11 to anedge 111. Formation of the second graphene electrode 16 into the secondgraphene electrode strip 160 can be executed by the formation of a mask,which can include polymethyl methacrylate (PMMA) resist material that ispatterned by electron-beam lithography techniques, and a subsequent etchof sections of the second graphene electrode 16 that are exposed by themask. The subsequent etch can be a RIE process. Following completion ofthe etch process, the mask can be removed by, for example dissolution inacetone for PMMA resist material. The mask 15 protects the firstgraphene electrode strip 130 during the processing of the secondgraphene electrode strip 160.

A second conductive contact 17 is then deposited on the end section 162of the second graphene electrode strip 160. The second conductivecontact 17 can include a material selected from the group consisting oftitanium (Ti), palladium (Pd) and gold (Au). The second conductivecontact 17 can be patterned and deposited on the end section 162 by, forexample, PMMA-based lift-off lithography. As shown in FIG. 8, a width ofthe second graphene electrode strip 160 is less than respectivecorresponding widths of the first and second conductive contacts 12 and17, the ZnO block 14 and the mask 15. Formation of the second conductivecontact 17 completes a formation or assembly of photodetector 20 as avisibly transparent, UV photodetector.

With reference to FIG. 9, an operation of the photodetector 20 isillustrated. As shown in FIG. 9, visible light of varying wavelengthsand UV light is incident on the photodetector 20. As the substrate 10,the first and second graphene electrode strips 130 and 160 and the ZnOblock 14 are all transparent to visible light, the visible light of thevarying wavelengths passes through the photodetector 20. Conversely,because the ZnO block 14 is able to absorb the UV light, the UV lightdoes not pass through the photodetector 20 and instead is used in thegeneration of charge carriers that can be used in turn for the detectionof UV light.

Thus, the photodetector 20 can be incorporated into a device, such as asunshade, sunglasses or an optical element, that is actuatable in thepresence of UV light. Such a device could include a processing unitwhich is configured to receive current from the photodetector 20 andwhich is configured to determine from that current that UV light isincident on the photodetector 20. The device could further include aservo control unit, which is coupled to the processing unit and which isoperable by the processing unit to take an action relative to theincident UV light in accordance with the determination.

With reference to FIG. 10, a method of fabricating the photodetector 20described above is provided. As shown in FIG. 10, the method includeslaying the first graphene electrode 13 between substrate surface 11(block 1001), and depositing the first conductive contact 12 on thefirst graphene electrode 13 proximate to the edge 110 (block 1002). Themethod also includes patterning the ZnO 14 block over an end of thefirst graphene electrode 13 and proximal (i.e., nearby and surrounding)portions of the substrate surface 11 at a distance from the firstconductive contact 12 (block 1003). In some cases, the patterning of theZnO block can be preceded by a deposition of a seed layer to promotenucleation (block 1004). Respective complementary sections 120 and 140of the first conductive contact 12 and the ZnO block 14 are then masked(block 1005) as is the region 1214 defined between the respectivecomplementary sections 120 and 140 (block 1006). Next, the secondgraphene electrode 16 is laid onto an unmasked section of the ZnO block14 with end section 162 of the second graphene electrode 16 being laidonto the substrate surface 11 (block 1007). The second conductivecontact 17 is deposited onto the end section 162 of the second grapheneelectrode 16 (block 1008).

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of this invention. Althoughvarious connections and positional relationships (e.g., over, below,adjacent, etc.) are set forth between elements in the followingdescription and in the drawings, persons skilled in the art willrecognize that many of the positional relationships described herein areorientation-independent when the described functionality is maintainedeven though the orientation is changed. These connections and/orpositional relationships, unless specified otherwise, can be direct orindirect, and the present invention is not intended to be limiting inthis respect. Accordingly, a coupling of entities can refer to either adirect or an indirect coupling, and a positional relationship betweenentities can be a direct or indirect positional relationship. As anexample of an indirect positional relationship, references in thepresent description to forming layer “A” over layer “B” includesituations in which one or more intermediate layers (e.g., layer “C”) isbetween layer “A” and layer “B” as long as the relevant characteristicsand functionalities of layer “A” and layer “B” are not substantiallychanged by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “exemplary” is used herein to mean “serving as anexample, instance or illustration.” Any embodiment or design describedherein as “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs. The terms “at least one”and “one or more” are understood to include any integer number greaterthan or equal to one, i.e. one, two, three, four, etc. The terms “aplurality” are understood to include any integer number greater than orequal to two, i.e. two, three, four, five, etc. The term “connection”can include an indirect “connection” and a direct “connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the described structures andmethods, as oriented in the drawing figures. The terms “overlying,”“atop,” “on top,” “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements such as an interfacestructure can be present between the first element and the secondelement. The term “direct contact” means that a first element, such as afirst structure, and a second element, such as a second structure, areconnected without any intermediary conducting, insulating orsemiconductor layers at the interface of the two elements.

Spatially relative terms, e.g., “beneath,” “below,” “lower,” “above,”“upper,” and the like, can be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” can encompass both an orientation ofabove and below. The device can be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

The phrase “selective to,” such as, for example, “a first elementselective to a second element,” means that the first element can beetched and the second element can act as an etch stop.

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

The term “conformal” (e.g., a conformal layer) means that the thicknessof the layer is substantially the same on all surfaces, or that thethickness variation is less than 15% of the nominal thickness of thelayer.

The terms “epitaxial growth and/or deposition” and “epitaxially formedand/or grown” mean the growth of a semiconductor material (crystallinematerial) on a deposition surface of another semiconductor material(crystalline material), in which the semiconductor material being grown(crystalline overlayer) has substantially the same crystallinecharacteristics as the semiconductor material of the deposition surface(seed material). In an epitaxial deposition process, the chemicalreactants provided by the source gases can be controlled and the systemparameters can be set so that the depositing atoms arrive at thedeposition surface of the semiconductor substrate with sufficient energyto move about on the surface such that the depositing atoms orientthemselves to the crystal arrangement of the atoms of the depositionsurface. An epitaxially grown semiconductor material can havesubstantially the same crystalline characteristics as the depositionsurface on which the epitaxially grown material is formed. For example,an epitaxially grown semiconductor material deposited on a {100}orientated crystalline surface can take on a {100} orientation. In someembodiments of the invention, epitaxial growth and/or depositionprocesses can be selective to forming on semiconductor surface, andcannot deposit material on exposed surfaces, such as silicon dioxide orsilicon nitride surfaces.

As previously noted herein, for the sake of brevity, conventionaltechniques related to semiconductor device, integrated circuit (IC)fabrication and photodetector fabrication may or may not be described indetail herein. By way of background, however, a more general descriptionof the semiconductor device, IC or photodetector fabrication processesthat can be utilized in implementing one or more embodiments of thepresent invention will now be provided. Although specific fabricationoperations used in implementing one or more embodiments of the presentinvention can be individually known, the described combination ofoperations and/or resulting structures of the present invention areunique. Thus, the unique combination of the operations described inconnection with the fabrication of a semiconductor device, an IC or aphotodetector according to the present invention utilize a variety ofindividually known physical and chemical processes performed on asemiconductor (e.g., silicon), glass or plastic substrate, some of whichare described in the immediately following paragraphs.

In general, the various processes used to form semiconductor-baseddevices (e.g., a photodetector) fall into four general categories,namely, film deposition, removal/etching, semiconductor doping andpatterning/lithography. Deposition is any process that grows, coats, orotherwise transfers a material onto the wafer. Available technologiesinclude physical vapor deposition (PVD), chemical vapor deposition(CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE)and more recently, atomic layer deposition (ALD) among others.Removal/etching is any process that removes material from the wafer.Examples include etch processes (either wet or dry), andchemical-mechanical planarization (CMP), and the like. Semiconductordoping is the modification of electrical properties by doping, forexample, transistor sources and drains, generally by diffusion and/or byion implantation. These doping processes are followed by furnaceannealing or by rapid thermal annealing (RTA). Annealing serves toactivate the implanted dopants. Films of both conductors (e.g.,poly-silicon, aluminum, copper, etc.) and insulators (e.g., variousforms of silicon dioxide, silicon nitride, etc.) are used to connect andisolate transistors and their components. Selective doping of variousregions of the semiconductor substrate allows the conductivity of thesubstrate to be changed with the application of voltage. By creatingstructures of these various components, millions of transistors can bebuilt and wired together to form the complex circuitry of a modernmicroelectronic device. Semiconductor lithography is the formation ofthree-dimensional relief images or patterns on the semiconductorsubstrate for subsequent transfer of the pattern to the substrate. Insemiconductor lithography, the patterns are formed by a light sensitivepolymer called a photo-resist. To build the complex structures that makeup a transistor and the many wires that connect the millions oftransistors of a circuit, lithography and etch pattern transfer stepsare repeated multiple times. Each pattern being printed on the wafer isaligned to the previously formed patterns and slowly the conductors,insulators and selectively doped regions are built up to form the finaldevice.

The flowchart and block diagrams in the Figures illustrate possibleimplementations of fabrication and/or operation methods according tovarious embodiments of the present invention. Variousfunctions/operations of the method are represented in the flow diagramby blocks. In some alternative implementations, the functions noted inthe blocks can occur out of the order noted in the Figures. For example,two blocks shown in succession can, in fact, be executed substantiallyconcurrently, or the blocks can sometimes be executed in the reverseorder, depending upon the functionality involved.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments described. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A method of fabricating a photodetector, themethod comprising: laying a first single-layer graphene electrode onto asubstrate; patterning a visibly transparent block over an end of thefirst single-layer graphene electrode and portions of the substrate suchthat an edge of the visibly transparent block and an edge of the firstsingle-layer graphene electrode are not coincident; and laying a secondsingle-layer graphene electrode onto an unmasked section of the visiblytransparent block with an end of the second single-layer grapheneelectrode laid onto the substrate.
 2. The method according to claim 1,wherein: the substrate is visibly transparent; and the method furthercomprises depositing first and second conductive contacts comprisingmaterials selected from the group consisting of titanium (Ti), palladium(Pd) and gold (Au) onto respective opposite ends of the first and secondsingle-layer graphene electrodes.
 3. The method according to claim 1,wherein the first and second single-layer graphene electrodes are dopedwith n-type or p-type dopants.
 4. The method according to claim 1,wherein laying the first and second single-layer graphene electrodescomprises: executing a process selected from the group consisting ofmechanically exfoliating, transferring and epitaxially growing the firstand second single-layer graphene electrodes; and etching the first andsecond single-layer graphene electrodes to form first and secondsingle-layer graphene electrode strips.
 5. The method according to claim1, wherein patterning the block, masking the section of the visiblytransparent block, and masking the exposed sections of the firstsingle-layer graphene electrode each comprises lithographic patterningand deposition.
 6. The method according to claim 1 further comprisingdepositing a seed layer over the end of the first single-layer grapheneelectrode and the portions of the substrate prior to patterning thevisibly transparent block.
 7. A method of fabricating a visiblytransparent, ultraviolet (UV) photodetector, the method comprising:laying a first single-layer graphene electrode between a substrate and afirst contact; patterning a visibly transparent block over an end of thefirst single-layer graphene electrode and proximal portions of thesubstrate at a distance from the first contact; masking complementarysections of the first contact and the visibly transparent block; maskinga region defined between the complementary sections of the first contactand the visibly transparent block; laying a second single-layer grapheneelectrode onto an unmasked section of the visibly transparent block withan end of the second single-layer graphene electrode being laid onto thesubstrate; and assembling a second contact onto the end of the secondsingle-layer graphene electrode.
 8. The method according to claim 7,wherein: the substrate is visibly transparent, and the first and secondcontacts comprise a material selected from the group consisting oftitanium (Ti), palladium (Pd) and gold (Au).
 9. The method according toclaim 7, wherein the first and second single-layer graphene electrodesare doped with n-type or p-type dopants.
 10. The method according toclaim 7, wherein the laying of the first single-layer graphene electrodebetween the substrate and the first contact comprises: a processselected from the group of processes consisting of mechanicallyexfoliating, transferring and epitaxially growing the first single-layergraphene electrode; and etching the first single-layer grapheneelectrode into a first single-layer graphene electrode strip; andlithographically depositing the first contact onto an end of the firstsingle-layer graphene electrode strip.
 11. The method according to claim7, wherein the patterning of the visibly transparent block, the maskingof the complementary sections of the first contact and the visiblytransparent block and the masking of the region between thecomplementary sections of the first contact and the visibly transparentblock each comprises lithographic patterning and deposition.
 12. Themethod according to claim 7, further comprising depositing a seed layerover the end of the first single-layer graphene electrode and theproximal portions of the substrate prior to the patterning of thevisibly transparent block.
 13. The method according to claim 7, whereinthe laying of the second single-layer graphene electrode comprises: aprocess selected from the group of processes consisting of mechanicallyexfoliating, transferring and epitaxially growing the secondsingle-layer graphene electrode; and etching the second single-layergraphene electrode into a second single-layer graphene electrode strip.